For the iPhone 5 the invites went out September 4 and the event was on the 12th.
4S event invites went out September 27 and the event as on October 4.
The 3G through the 4 were all released at WWDC so that date was known quite a bit in advance, and the original iPhone was released at Macworld so it was known that there would be an event then for quite some time.
The invites seem to go out on Tuesdays about a week before the event so I would say if the event is actually happening on the 10th which it seems to be the invites will go out September 3.